1. Field of the Invention
The present invention relates to a time delay apparatus for a system which requires time delay of a signal, and, more particularly, the present invention relates to a time delay apparatus including a transfer conductance portion which uses transfer conductance to control a signal from an external device and obtain a desirable time delay without influencing a frequency characteristic, by using a MOS (Metal-Oxide Semiconductor) transistor.
2. Description of the Related Art
Generally, in signal processing system, time delay of a signal is inevitably generated in a path between components. For example, a signal passing through a simple buffer generates a time delay of at least 2 nanoseconds (ns). Further, for example, a time delay dependent on holding time is generated in a sample-and-hold circuit installed in an analog to digital converter (A/D converter) for converting an analog signal to a digital signal.
However, in a system for processing an inputted multiplexed signal by using the time delay characteristics of each signal, the time delay between signals is different because the path for inputting and passing through each signal generated from an external device is not the same. Accordingly, adding a time delay according to the path through which the signal passes to the time characteristics of an ordinarily generated signal can produce a bad influence on the system.
A conventional time delay apparatus will be described hereinbelow with reference to a time delay control apparatus applied to a digital video disk player (DVDP) which is a system for processing a multiplexed signal and standardized as an alternative of a new video and voice storing device.
A block diagram of the conventional digital video disk player is shown in FIG. 1. The digital video disk player includes: a data collecting portion 10 for collecting data from a high-density optical disk via a head; a data signal processing portion 20 which outputs the data as a bit stream by executing demodulation and error correction of the collected data from the data collecting portion 10 and feeds back the data to the data collecting portion 10 by detecting a tracking error; a signal reproducing portion 30 for generating audio and video outputs by decoding each audio, character and video signal among the bit streams inputted from the data signal processing portion 20; and a system control portion 40 for inputting a control signal for display and operation and a control signal for a whole system into the data signal processing portion 20 and the signal reproducing portion 30, thereby providing a user interface.
The data collecting portion 10 includes: an optical head 12 for directly collecting data from the high-density optical disk 14 which records data along a track; a disk motor 11 for rotating the optical head 12 at a constant speed; and a transporting motor 13 for transporting the optical head 12 to an exact position on the optical disk 14.
Moreover, the data signal processing portion 20 includes: a signal amplifying portion 21 for outputting the signal received from the data collecting portion 10 as a safe signal by amplifying the same; a demodulating/error correcting portion 22 for applying the inputted signal from the signal amplifying portion 21 to the signal reproducing portion 30 as a bit stream by demodulating the inputted signal from the signal amplifying portion 21 and detecting and recovering the error generated during recording of the signal on the optical disk 14 by error correcting code; a servo control portion 23 which generates a track control signal of the data collecting portion 10 by using the track error of the data collecting portion 10 which is detected by using four (4) signals, such as a first, a second, a third and a fourth signal inputted from the signal amplifying portion 21, and a control signal inputted from the demodulating/error correcting portion 22; and a servo driving portion 24 for converting the track control signal inputted from the servo control portion 23 to a motor driving signal of the data collecting portion 10.
The signal reproducing portion 30 includes: a system decoding portion 31 for separating the inputted signal bit stream from the demodulating/error correcting portion 22 of the signal processing portion 20 into video, character and audio signals according to data characteristics; a video decoding portion 32 for decoding the separated video signal stream from the system decoding portion 31; a character decoding portion 33 for encoding the separated character signal stream from the system decoding portion 31; an audio decoding portion 34 for decoding the separated audio signal stream from the system decoding portion 31; a National Television Standard Committee (NTSC) decoding portion 35 for converting the inputted signal from the video decoding portion 32 and the character decoding portion 33 into a NTSC signal which is a displaying method of a broadcasting and a video player; and a Digital to Analog (D/A) converter 36 for converting the inputted signal from the audio decoding portion 34 into the audio signal.
The servo-control portion 23 will now be described in detail hereinbelow. The servo-control portion 23 receives and processes a multiplexed signal from the signal amplifying portion 21 and the demodulating/error correcting portion 22. That is, to execute track error detection for a beam used for the digital video disk player, the four signals from the signal amplifying portion 21 are received and the phase difference between a first composite signal and a second composite signal is detected, wherein the first composite signal is a composite of the first and the third signals and the second composite signal is the composite of the second and the fourth signals. Accordingly, the signal track on the optical disk 14 is discriminated.
Then, respectively different time delays can be generated by an offset of a pickup process and a delay component of the signal amplifying portion 21, etc., because each signal uses respectively different channels.
Accordingly, when the mechanism for correcting the exact position of the optical head portion 12 causes error because of the phase difference generated from the track error and the time delay according to the offset of the pickup process and the delay component of the signal amplifying portion 21, deviation from the track can be generated at the servo-control portion 23 which detects the phase difference between the first composite signal and the second composite signal. Consequently, deviation from the track may cause discontinuous data collecting.
In addition to the above-described time delay, a time delay of the signal is needed for the general signal processing system for processing a single signal. The time delay can be obtained by using various methods, such as using a passive component or an active component. Generally, the time delay method can be roughly classified into a method using the passive component and a method using the active component.
The most simple time delay method using a passive element is to use a capacitor. By using the capacitor characteristics, that is, the capacitor charges to a predetermined value according to an inputted signal and discharges when the inputted signal is eliminated and the speed of charge and discharge is determined by the capacitance of the capacitor. Further, the time constant is changed by changing the capacitance of the capacitor and the required time delay can be obtained via change of the time constant.
Moreover, the active delay component having a naturally discrete time delay characteristic, such as a flip-flop, can delay time by continuously connecting the same in a serial manner. In the time delay method using a flip-flop, the degree of the time delay is determined by a the frequency of a clock applied to the delay component. By changing the clock frequency of an oscillator, the degree of the time delay is controllable.
The time delay method using the active delay component is preferably used for a system requiring a discrete time delay, and a required time delay signal between each flip-flop can be output by using the signal from an external device when the active delay component is installed on one chip.
Another time delay method using the active component is to use an all-band pass filter. To obtain characteristics of the all-band pass filter, a low pass filter (LPF) consisting of two operational amplifiers is used so that the LPF obtains two poles and two roots. By using the LPF, the parameter of the all-band pass filter having a transfer function, such as Function 1 below is determined. ##EQU1##
In the Function 1, a is a primary coefficient of the denominator polynomial expression of the transfer function, b is a constant of the denominator polynomial expression of the transfer function, c is a primary coefficient of the numerator polynomial expression of the transfer function and d is a constant of numerator polynomial expression of the transfer function.
Generally, for forming the low frequency pass filter of the all-band pass filter, a resistance component and a capacitor component are required. To obtain a required amount of the time delay at the inputted signal, the resistance component and the capacitor component are changed.
When the capacitance of the capacitor component is determined, a time delay amount is fixed in the time delay method using the capacitor component. To obtain the exactly required amount of the time delay, the precise capacitance of the capacitor component is changed via trial and error. When the time delay function is provided in one chip, it is difficult to change the amount of the time delay from the outside.
Moreover, the time delay method using the active delay component has some problems. For example, it is difficult to determine the degree of the time delay actively because the degree of the time delay is fully determined by the frequency of a clock used for the delay component. The resolution of the time delay cannot be improved to a predetermined level because of a limit of the clock speed and the characteristics of the active delay component. The active time delay component itself inevitably generates the time delay. To obtain an exact time delay, an additional means is required because the time delay method using the active delay component is only applied to the discrete signal.
Consequently, the degree of the time delay is fixed and, accordingly, it is difficult to control the time delay from the outside in the conventional time delay method. Moreover, in the conventional time delay method, the resolution of the time delay is restricted.